Purging of Cold Vent

[cubicsupport]

I am looking at the need to purge the cold vent line with nitrogen to avoid possible exploxive mixture in the cold vent header. At present the process (Methane)blowdown is connected to the cold vent header. We previously used fuel gas as purge gas to avoid air ingress into the header. I have seen installation where there is no purge. I was lead to believe that the volume of the header is a factor in deciding if purging is required.

Any there any gudeline?

 

[dcasto]

Vent or Flare header?

The choice to have a purge depends on whats in the header. If you have a poisionous gas the a purge would be required. If you are required to open the header for maintenance on things connected, then a purge would be required. If you have no seal on the outlet, then a purge will be required.

If you purge with N2 and it is a flare, then you will be adding methane for fuel at some point, and the amount of methae to add goes up with more N2 in the header.

Think about these things to get started. Others will add to the list.

 

[Flareman]

Cubicsupport
Like dcasto, I'm just a little confused by your question but I'm going to conclude that this is a plain vent with no supplementary means of ignition. Your need to purge is precautionary, after the flow of flammable gas has stopped, such that you don't decant the flammable gas out and some air in, giving you a nasty explosive mixture (although I guess that the previous use of fuel gas didn't necessarily avoid that problem).

The best way to approach this is to treat the vent as though it were a flare, because if it ignites, it will be. I have some information on purging at

http://www.geocities.com/flareman_xs

but you might also want to drop me an email through that site and I'll send you a couple of papers with more informaion.

The bottom line is that a small constant flow of flammable gas will keep a "safe" oxygen profile (as a funtion of diameter and gas properties). If you use Nitrogen, the rate you need is less and, depending on how you relieve process gas, you might be able to just post-purge one time after each event. This will work if the future reliefs are all either predictable or at a high rate. If you can get a dribble of flammable gas, then you will need a constant flow, even of nitrogen.

David

 

[cubicsupport]

Thanks Dcasto/flareman for the info.

At present we have 2 different cold flare systems on our remote unmanned gas platforms(sweet gas). The old platforms approx 1990, have a fuel gas purge on the cold flare header.The vents are for topside blowdown and are 8" NB.
On the newer installation, 2002, the vent header is 3"NB and does not have a purge at all.
In each case the vent header lengths are approximately the same, but the volumes are different due to the different diameters. Previously, I was lead to believe that if the volume of vent header was "small", then there is no need to purge as the "explosion" could be contained within the pipework. I have no documented basis for this. Previous threads have also indicated that the velocity of discharge during blowdown will force any air ingress out of the vent header pipe. As the vent is for blowdown, it will go out as soon as the topside is depressurised and the blowdown valves close.

As part of cost cutting, we need to possibly eliminate the need for nitrogen on the platform.

 

[Flareman]

Cubicsupport
It is certainly true that some pipes will be intrinsically strong enough to withstand a potential detonation. For the sake of a design check, think 700 - 800 psi as a momentary pressure.

In thread798-196567 I offer an opinion about a possible approach to pressure design against detonation.

David